1. Field of the Invention
The present invention relates to optical element that changes the phase of an incident light beam and to an optical head and an optical recording and reproducing apparatus using such an optical element.
2. Description of the Prior Art
Optical recording media, such as digital versatile disks (DVDs), can record digital information with high densities, so that they are noted as optical recording media with large capacities. To record and reproduce digital information with high densities, it is necessary to use short wavelength light for recording and reproducing, and make the NA (numerical aperture) of the objective lens large. However, making the wavelength short and the NA of the lens large increases the wave-front aberrations, in particular coma aberration, due to deviations (tilt) from the optical axis, for example caused by warps in the optical recording medium, and there is the problem that the system design margins for the tilt have to be reduced.
In order to solve this problem, optical heads correcting the wave-front aberrations with a liquid crystal panel have been proposed (see e.g. Japanese Patent Application (Tokkai) Hei 9-128785).
Referring to FIG. 23, the following explains an example of such a conventional head.
FIG. 23 shows the configuration of a conventional optical head 1 (also called xe2x80x9coptical pickupxe2x80x9d). The optical head 1 comprises a light source 2, a half-mirror 3a, an objective lens 3b, a focus lens 3c, an optical element 4, a tilt sensor 5, an optical element control circuit 6, and a photo-detector 7.
The light source 2, which can be a semiconductor laser element, outputs coherent light for recording and reproducing toward the recording layer of the optical recording medium 8 (which is a medium for recording information, in which the recorded information can be read out optically, such as a CD or a DVD). The optical element 4 includes a liquid crystal panel, which has a plurality of segment electrodes in a pattern such as shown in FIG. 24. By applying an appropriate voltage to each of the segment electrodes, the optical element 4 changes the refractive index of the liquid crystal for each of the segment electrodes, and changes the phase of the light passing through each of the electrodes. Thus, the optical element 4 corrects the aberration caused by the tilt.
The following explains how this conventional optical head 1 functions. Linearly polarized light emitted by the light source 2 is reflected by the half-mirror 3a and enters the optical element 4. When the optical recording medium 8 is tilted vertically with respect to the optical axis, a signal depending on the tilt amount (tilt angle) is output by the tilt sensor 5. Based on the signal from the tilt sensor 5, the optical element control circuit 6 controls the liquid crystal panel of the optical element 4 so as to generate the necessary phase change for correcting the wave-front aberrations caused when the optical recording medium is tilted.
Thus, the light entering the optical element 4 is subjected to a phase change that corrects the wave-front aberrations caused by tilting of the optical recording medium 8. The light that has passed through the optical element 4 is focused on the optical recording medium 8 by an objective lens 3b. Since light that has been subjected to a phase change that corrects the wave-front aberrations caused by tilting of the optical recording medium 8 is focused by the objective lens 3b, a light spot without aberration (constricted to the diffraction limit) is formed on the optical recording medium 8.
Then, the light that is reflected from the optical recording medium 8 may turn into light with wave-front aberrations, depending on the tilt of the optical recording medium 8, but the wave-front aberrations are corrected by the optical element 4. The light that has passed through the optical element 4 passes the half-mirror 3a and enters the focus lens 3c without returning to the light source 1, and is focused by the focus lens 3c on the photo-detector 7. The photo-detector 7 outputs the information stored on the optical recording medium 8. In addition, the photo-detector 7 outputs a focus error signal indicating how well the light focuses on the optical recording medium 8, and a tracking error signal indicating the irradiation position of the light.
The following explains the principle of the tilt correction with the optical element 4.
FIG. 25 shows an example of the wave-front aberrations in the best image point of the optical recording medium 8 (for a 1xc2x0 tilt angle of the optical recording medium 8, a 0.6 NA of the objective lens, a wavelength of 655 nm, and a 0.6 mm substrate thickness of the optical recording medium 8). As is shown in FIG. 25, if the optical recording medium 8 is tilted, the wave-front aberrations have a substantially semicircular distribution, anti-symetrically to the left and right. By subjecting the incident light to a phase change that cancels the wave-front aberration distribution in FIG. 25 with the optical lens 4, the spot on the optical recording medium 8 can be constricted to the diffraction limit, even when the optical recording medium 8 is tilted. Moreover, by subjecting the light that is reflected from the optical recording medium 8 to a phase change that cancels the wave-front aberrations, the photo-detection with the photo-detector 7 becomes more precise.
To subject the incident light to a phase change that cancels the wave-front aberration distribution in FIG. 25, the light path length of the optical element 4 has to be changed partially. Since the refractive index of the liquid crystal can be changed by applying a voltage from outside, the light path length of the optical element can be changed partially by partially changing the applied voltage. Thus, the wave-front aberrations shown in FIG. 25 can be corrected by applying different voltages to different segment electrodes partitioned into a fine pattern as shown in FIG. 24.
However, with such an optical element 4, it is necessary to apply a corresponding control signal from the outside to each segment electrode of the liquid crystal panel in the optical element 4. This means, that from the driving circuit for driving the liquid crystal panel, a flexible circuit board has to be connected to the optical element 4 with the same number of wires as there are segment electrodes in the liquid crystal panel. Consequently, in the case of an optical element 4 having many segment electrodes as shown in FIG. 24, many signals have to be supplied, and the flexible circuit board becomes accordingly wider. When such a wide flexible circuit board is connected to the optical element 4, it becomes very difficult to adjust the parts properly, and this stands in the way of making the optical head 1 smaller. Moreover, to install a small optical element as the optical head 4 without shorting the many wires is very difficult, and the more wires there are, the worse becomes the yield of the step of installing the optical element 4 on the flexible circuit board, which leads to higher costs for the optical head 1.
In order to solve this problem, an optical element has been proposed having segment electrodes that differ from the above conventional segment electrodes (Tokkai Hei 10-20263). FIG. 26 shows the shape of these segment electrodes. The shape of these segment electrodes agrees with the shape of the wave-front aberrations caused by tilting of the optical recording medium 8. Consequently, the wave-front aberrations can be corrected better than with the above-mentioned conventional optical element, even though the number of segment electrodes is reduced.
However, even with the optical element having segment electrodes of the shape shown in FIG. 26, the pattern shown in FIG. 26 has to be partitioned even finer to correct the wave-front aberrations more precisely. Consequently, even in an optical element having segment electrodes with the shape shown in FIG. 26, the number of control signals rises in proportion to the partitioning degree of the segment electrodes, and there is the problem that it becomes difficult to connect the optical element 4 and the optical element control circuit 6. Moreover, there is the problem that it is difficult to make the optical head smaller.
Moreover, the influence of the electric field above the portions between segment electrode and segment electrode (separation portions) is weak, so that there is the problem that the wave-front aberrations are not corrected sufficiently. It seems possible to make the separation portions narrower, but if the width of the separation portions is below a certain value, there is the problem that they become difficult to manufacture and the yield drops.
To solve these problems, it is an object of the present invention to present an optical element with a good correcting effect with regard to incident light, which is easy to manufacture, and an optical head and an optical recording and reproducing apparatus using the same.
To attain these objects, an optical element in accordance with the present invention includes a voltage applying electrode provided with a plurality of segment electrodes; an opposing electrode arranged substantially in parallel to the voltage applying electrode and opposing the voltage applying electrode; a phase changing layer made of a phase changing material arranged between the voltage applying electrode and the opposing electrode; wherein a phase of light entering the phase changing layer is changed by changing a voltage between the voltage applying electrode and the opposing electrode. With this configuration, an optical recording and reproducing apparatus with good correcting effect with regard to incident light, which is easy to manufacture, can be achieved.
In the optical element, it is preferable that the phase changing material is a material whose refractive index changes depending on the voltage. With this configuration, the phase of the incident light can be changed easily.
In the optical element, it is preferable that the phase changing material is a liquid crystal. With this configuration, the voltage applied to change the phase of the incident light can be small.
In the optical element, it is preferable that the phase changing material is a material whose volume changes depending on the voltage. With this configuration, the phase of the incident light can be changed easily.
In the optical element, it is preferable that the phase changing material is PLZT (transparent perovskite crystal containing lead oxide, lanthanum, zirconium oxide, and titanium oxide). With this configuration, the element can be made thin.
It is preferable that the voltage applying electrode further includes a voltage control electrode made of a conductive material, which divides a voltage applied from outside with resistances of the conductive material and applies the divided voltages to the segment electrodes. With this configuration, the resistances of the voltage applying electrodes divide a voltage applied from outside, so that the voltage applied from outside can be divided easily.
In the optical element, it is preferable that the plurality of segment electrodes includes a plurality of substantially semicircular electrodes arranged substantially in symmetry. With this configuration, the wave-front aberrations can be corrected easily and precisely.
In the optical element, it is preferable that the plurality of segment electrodes includes a plurality of electrodes partitioned into concentric rings. With this configuration, spherical aberration can be corrected easily and precisely.
In the optical element, it is preferable that the thickness da of the voltage applying electrode is
da=(2Na+1)xcex/2na
wherein xcex is the wavelength of the incident light, Na is an integer of 0 or greater, and na is the refractive index of the voltage applying electrode. With this configuration, light reflections at the voltage applying electrode can be prevented, if the refractive indices of the layer above and below the voltage applying electrode are substantially the same.
In the optical element, it is preferable that the thickness db of the opposing electrode is
db=(2Nb+1)xcex/2nb
wherein xcex is the wavelength of the incident light, Nb is an integer of 0 or greater, and nb is the refractive index of the opposing electrode. With this configuration, light reflection at the opposing electrode can be avoided, if the refractive indices of the layer above and below the opposing electrode are substantially the same.
It is preferable that the optical element further includes an anti-reflective film for preventing reflection of incident light. With this configuration, light losses due to reflection can be prevented.
In the optical element, it is preferable that the voltage applying electrode is partitioned by separation portions into the plurality of segment electrodes; and the width of the separation portions is such that the entire phase changing layer located above the separation portions is influenced by the segment electrodes. With this configuration, the phase of light passing through the portions where the segment electrode is not formed can be controlled as well, so that an optical element with a particularly high phase correction effect can be obtained.
In this optical element, it is preferable that the width W of the separation portions and the thickness d of the phase changing layer satisfy the relation Wxe2x89xa63d. With this configuration, an optical element with an even better phase correction effect can be obtained.
In the optical element, it is preferable that the voltage applying electrode is partitioned by separation portions into the plurality of segment electrodes; and the optical element further includes a light-blocking film blocking light from passing through the separation portions. With this configuration, light passing through the portions of the liquid crystal that are not controlled by the electric field can be blocked, so that an optical element with an even better phase correction effect can be obtained.
In this optical element, it is preferable that the light-blocking film is made of metal. With this configuration, a light-blocking film that blocks light well can be manufactured easily.
An optical head in accordance with the present invention, for optically reading information stored on an optical recording medium, includes a light source and an optical element as described above arranged between the optical recording medium and the light source. Since this optical head includes an optical element in accordance with the present invention, an optical head with good correcting effect with regard to incident light, which is easy to manufacture, can be achieved.
In this optical head, it is preferable that the phase changing material of the optical element is a material whose refractive index changes depending on the voltage.
In this optical head, it is preferable that the phase changing material is a liquid crystal.
In this optical head, it is preferable that the phase changing material of the optical element is a material whose volume changes depending on the voltage.
In this optical head, it is preferable that the phase changing material is PLZT.
In this optical head, it is preferable that the voltage applying electrode further includes a voltage control electrode made of a conductive material, which divides a voltage applied from outside with resistances of the conductive material and applies the divided voltages to the segment electrodes.
In this optical head, it is preferable that the plurality of segment electrodes includes a plurality of substantially semicircular electrodes arranged substantially in symmetry.
In this optical head, it is preferable that the plurality of segment electrodes includes a plurality of electrodes partitioned into concentric rings.
In this optical head, it is preferable that the thickness da of the voltage applying electrode is
da=(2Na+1)xcex/2na
wherein xcex is the wavelength of the incident light, Na is an integer of 0 or greater, and na is the refractive index of the voltage applying electrode.
In this optical head, it is preferable that the thickness db of the opposing electrode is
db=(2Nb+1)xcex/2nb
wherein xcex is the wavelength of the incident light, Nb is an integer of 0 or greater, and nb is the refractive index of the opposing electrode.
In this optical head, it is preferable that the optical element further includes an anti-reflective film for preventing reflection of incident light.
In this optical head, it is preferable that the voltage applying electrode is partitioned by separation portions into the plurality of segment electrodes; and the width of the separation portions is such that the entire phase changing layer located above the separation portions is influenced by the segment electrodes.
In this optical head, it is preferable that the width W of the separation portions and the thickness d of the phase changing layer satisfy the relation Wxe2x89xa63d.
In this optical head, it is preferable that the voltage applying electrode is partitioned by separation portions into the plurality of segment electrodes; and the optical element further includes a light-blocking film formed on a main surface on the outer side of the first substrate or the second substrate at a position corresponding to the separation portions.
In this optical head, it is preferable that the light-blocking film is made of metal.
It is preferable that the optical head further includes a N xcex/4 plate (wherein N is an odd number of one or greater) arranged between the optical recording medium and the optical element. With this configuration, light is used with greater efficiency, facilitating the recording of signals.
An optical recording and reproducing apparatus in accordance with the present invention, for recording or reproducing signals (this includes recording and reproducing signals) on an optical recording medium, includes an optical head for recording or reproducing signals on the optical recording medium, the optical head including a light source, and an optical element as described above arranged between the optical recording medium and the light source. Since this optical recording and reproducing apparatus includes an optical element in accordance with the present invention, an optical recording and reproducing apparatus with a good correcting effect with regard to incident light, and which is easy to manufacture, can be achieved.
In the apparatus, it is preferable that the phase changing material of the optical element is a material whose refractive index changes depending on the voltage.
In the apparatus, it is preferable that the phase changing material is a liquid crystal.
In the apparatus, it is preferable that the phase changing material of the optical element is a material whose volume changes depending on the voltage.
In the apparatus, it is preferable that the phase changing material is PLZT.
In the apparatus, it is preferable that the voltage applying electrode further includes a voltage control electrode made of a conductive material, which divides a voltage applied from outside with resistances of the conductive material and applies the divided voltages to the segment electrodes.
In the apparatus, it is preferable that the plurality of segment electrodes includes a plurality of substantially semicircular electrodes arranged substantially in symmetry.
In the apparatus, it is preferable that the plurality of segment electrodes includes a plurality of electrodes partitioned into concentric rings.
In the apparatus, it is preferable that the thickness da of the voltage applying electrode is
da=(2Na+1)xcex/2na
wherein xcex is the wavelength of the incident light, Na is an integer of 0 or greater, and na is the refractive index of the voltage applying electrode.
In the apparatus, it is preferable that the thickness db of the opposing electrode is
db=(2Nb+1)xcex/2nb
wherein xcex is the wavelength of the incident light, Nb is an integer of 0 or greater, and nb is the refractive index of the opposing electrode.
It is preferable that the apparatus further includes an anti-reflective film for preventing reflection of incident light.
In the apparatus, it is preferable that the voltage applying electrode is partitioned by separation portions into the plurality of segment electrodes; and the width of the separation portions is such that the entire phase changing layer located above the separation portions is influenced by the segment electrodes.
In the apparatus, it is preferable that the width W of the separation portions and the thickness d of the phase changing layer satisfy the relation Wxe2x89xa63d.
In the apparatus, it is preferable that the voltage applying electrode is partitioned by separation portions into the plurality of segment electrodes; and the optical element further includes a light-blocking film blocking light from passing through the separation portions.
In the apparatus, it is preferable that the light-blocking film is made of metal.
It is preferable that the apparatus further includes a N xcex/4 plate (wherein N is an odd number of one or greater) arranged between the light source and the optical element.